Monitoring technology

ABSTRACT

A process for infrared spectroscopic monitoring of insitu compositional changes in a polymeric material comprises the steps of providing an elongated infrared radiation transmitting fiber that has a transmission portion and a sensor portion, embedding the sensor portion in the polymeric material to be monitored, subjecting the polymeric material to a processing sequence, applying a beam of infrared radiation to the fiber for transmission through the transmitting portion to the sensor portion for modification as a function of properties of the polymeric material, monitoring the modified infrared radiation spectra as the polymeric material is being subjected to the processing sequence to obtain kinetic data on changes in the polymeric material during the processing sequence, and adjusting the processing sequence as a function of the kinetic data provided by the modified infrared radiation spectra information.

This invention was made with Government support under Contract No.NASI-18420 awarded by NASA. The Government has certain rights in thisinvention.

This invention relates to monitoring technology, and more particularlyto technology for monitoring the process of polymeric materials and thelike.

Characteristics of polymeric materials are functions of processing orother environmental factors to which they are exposed. For example,thermosetting polymeric materials are typically exposed during a curingsequence to changing temperature and pressure conditions over a timeinterval which curing sequence produces compositional changes in thepolymeric material. Similar changes in molecular and crystal structuresoccur in the processing of thermoplastic materials. In thermoset matrixcomposites, for example, the ultimate properties of the part depend to asignificant extent upon the resin being fully cross-linked or cured.

Fiber reinforced composite materials possess desirable properties thatmake them attractive as structural materials for applications in theaerospace, automotive, marine and other industries, such desirableproperties including strength and stiffness values significantly higherthan comparable monolithic materials due to the reinforcing fibers; theability to use various materials in such composites includingpolymer-matrix, metal-matrix and ceramic-matrix compositions to providea wide range of mechanical, thermal and chemical properties and thelike, as well as the ability to tailor such composites to obtaindesirable properties, for example by changing the fiber or matrixmaterial, or by changing fiber orientation.

Thermoset laminated composites form excellent structures if appropriateprocess conditions are accurately known and rigorously followed.Unfortunately, in practice, it has been difficult to translateappropriate process conditions into production, the resulting productslacking reproducibility due to inadequate process control information,which leads to costly discards. Frequently a test coupon made of asimilar layup is processed simultaneously with the desired part and thetest coupon is subjected to destructive testing to determine degree ofcure, voids, etc. within the part. Unfortunately, the test coupon willfrequently pass but the part will not be up to specifications.Nondestructive testing techniques that have been employed includeinfrared spectroscopy technology for characterizing material at or nearthe surface, ultrasonic scanning, dielectric cure monitoring, acousticemission, X-radiography and thermochromic analysis. None of theseefforts have provided adequate process control information.

In accordance with one aspect of the invention, there is provided asystem for infrared spectroscopic monitoring of polymeric material toobtain kinetic data on changes in compositional characteristics of thepolymer during a processing sequence that includes a source of infraredradiation for generating a beam of infrared radiation, infrared spectrumanalyzing means, an infrared radiation transmission fiber that has aclad transmission portion and an unclad sensor portion adapted to beembedded in the polymeric material to be monitored, and means forcoupling the transmission fiber to the source to transmit a beam ofinfrared radiation through the fiber to the sensor portion and forcoupling the fiber to the infrared spectrum analyzing means foranalyzing the resulting spectra as the polymeric material is processedto provide kinetic information on the processing of the polymericmaterial.

Preferrably, the infrared analyzer includes Michelson interferometertype apparatus that produces a modulated beam which is transmitted alongthe IR transmitting link to the sensor as a broad band spectral input,the radiation is modified by multiple internal reflections within thesensor and the resulting modified spectrum radiation is transmitted toanalyzer for analysis.

The sensor preferably includes a chalcogenide glass such as arsenicsulfide or arsenic germanium selenide, a heavy metal fluoride glass suchas a mixture of zirconium, barium, lanthanum and aluminum fluorides, orpolycrystalline or single crystal material such as thallium bromoiodideor cesium iodide. Preferably, the sensor fiber has a diameter of atleast about one hundred micrometers but less than one millimeter and arefractive index greater than 1.7. In a particular embodiment, the fibersensor-transmitter has a length of at least about two meters with a cladtransmission portion and an unclad sensor section, both of which areembedded in the polymer material to be monitored. The overalltransmission losses of the fiber are preferably less than 5 dB per meterover an 1800-750 wavenumber bandwidth and the analyzer monitorsradiation over an 5000-300 wavenumber bandwidth.

In a particular system for monitoring the curing of a polymer-matrixcomposite of the type which employs fibers of material such as graphiteor boron and polymers of materials such as epoxies or polyimides, aplurality of sensors are embedded in uncured polymer material in regionsbetween layers of fibers and connected by their transmission fiberportions through appropriate glands of an autoclave system to a Fouriertransform infrared analyzer such as the Digilab FTS-60 system. Theautoclave system includes containment structure in which thepolymer-matrix composite is disposed and by means of which programmedtemperature and pressure conditions are applied to the laminate during aprocessing cycle of several hours duration during which kinetic changesin chemical, crystalline, molecular and like characteristics of thepolymer such as epoxide formation (for example, at 908 cm⁻¹), H--C═bands (for example, at 844 and 3096 cm⁻¹), aliphatic C--H bands (forexample, at 2921 cm⁻¹), aromatic C--H bands (for example, at 3062 cm⁻¹),--SO₂ -- bands (for example, at 1150 cm⁻¹), imidization characteristics(for example, at 1775 cm⁻¹), and anhydride formation (for example, at1853 cm⁻¹) wave numbers may be monitored and process parameters adjustedas a function of such real time kinetic information.

In accordance with another aspect of the invention, there is provided aprocess for infrared spectroscopic monitoring of insitu compositionalchanges in a polymeric material comprising the steps of providing anelongated infrared radiation transmitting fiber that has a transmissionportion and a sensor portion, embedding the sensor portion in thepolymeric material to be monitored, subjecting the polymeric material toa processing sequence, applying a beam of infrared radiation to thefiber for transmission through the transmitting portion to the sensorportion for modification as a function of properties of the polymericmaterial, and monitoring the modified infrared radiation spectra as thepolymeric material is being subjected to the processing sequence toobtain kinetic data on changes in the polymeric material during theprocessing sequence. Preferably, the process further includes the stepof adjusting the processing sequence as a function of the kinetic dataprovided by the modified infrared radiation spectra information.

Other features and advantages of the invention will be seen as thefollowing description of particular embodiments progresses, inconjunction with the drawings, in which:

FIG. 1 is diagram of a monitoring system in accordance with theinvention;

FIG. 2 is an enlarged diagrammatic view of a portion of thepolymer-matrix composite being processed in the system of FIG. 1;

FIGS. 3 and 4 are enlarged diagrammatic views of sensor fiber systems inaccordance with the invention; and

FIG. 5 is a diagram indicating an optical coupling arrangement betweenthe sensor-transmitter fiber and the associated infrared spectroscopicanalyzer equipment.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The composite processing system shown in FIG. 1 includes a pressure andtemperature controlled environment housing 10 in which autoclaveassembly 12 is disposed. Assembly 12 includes mold plate 14 and bagstructure 16 which houses laminate structure 20 that is to be processed.Structure 20 includes an alternating series of fiber layers 22 (whichmay, for example, be woven) and uncured resin layers 24 that aredisposed in stacked relation. Sensors 26 are embedded in resin layers 24and connected to infrared transmission fibers 28 as indicated in FIG. 2.Surrounding laminate structure 20 is flexible dam structure 30. Disposedbelow and between laminate structure 20 and mold plate 14 is peel ply 32and sacrificial ply 34. Above laminate structure 20 and enclosed by bag16 are release fabric 36, bleeder plies 38, breather plies 40 ad caulplate 42. Bag seal 44 extends around the periphery of the flexible damstructure 30, and edge bleeder structure 46 is disposed between seal 44and dam 30. Coupled to edge bleeder 46 via passages 48 is mold-ventingmanifold which is connected through coupling 52 to a pressure sourcewhich may apply vacuum or atmospheric pressure depending on theparticular application.

The IR transmission fibers to which sensors 26 are connected are bundled(as indicated at 54) externally of the laminate structure 20 to beprocessed and extend through seal glands 56 to infrared spectroscopyanalyzer apparatus that includes Michelson interferometer source 58 andreceiver analyzer 60 and may be a Digilab FTS-60 analyzer.

With reference to the diagrammatic view of FIG. 2, disposed in the resin24 between the fiber layers 22 are IR transmitting fibers 62 that have asensor portion 26 and integral IR transmission portions 28. As indicatedin FIG. 3, the optical fiber 62 has polymeric or other suitable cladding64 in transmission portions 28 and is unclad at the sensor portion 26 sothat portion is exposed and in direct contact with resin 24. Theradiation beam 66 from interferometer source 58 is applied to input end68 and the modified exit beam 70 at output end 72 is connected todetector and processing apparatus 60. In another embodiment, shown inFIG. 4, the fiber 62' includes a single transmitter section 28' withcladding 64', unclad sensor portion 26', and reflector structure 74 sothat the transmitted beam 66' as modified by absorbance at the sensor26' is reflected back through fiber 62' to the entrance end. In anotherembodiment, a composite fiber has a sensing section 26 of chalcogenideglass and transmitting sections 28 of heavy metal fluoride glass. Theoptimal resin contact length of the sensor 26 is a function of therefractive indices of the fiber and the resin material to be monitored,epoxy and polyimide resins typically having refractive indices in therange of 1.5 to 1.6.

Shown in FIG. 5 in diagrammatic form is a sensor-transmitter fiber 62with input coupling optics 80 that includes toroid 82 and cone 84 forcoupling the input beam 66 from the interferometer to the fiber 62, formultiple internal reflections and modification by absorbance at thesensor region 26, and similar coupling optics 88 that includes toroid 90and cone 92 for directing the output beam 70 to the detector 60.

In an illustrative polyimide composite processing cycle, the laminatestructure 20 in autoclave 12 is initially at room temperature. Thetemperature is gradually increased to 200° F. and held for one half hourafter which full vacuum is applied and the temperature is ramped up to470° F. at which time 250 pounds pressure is applied. The temperatureand pressure are held for a half hour and then the temperature is againramped up to about 630° F., the laminate structure being continued to beheld under pressure for three hours and then the system cooled to lessthan 200° F. with vacuum vented and pressure released, the total cycleduration being about six hours. During this processing sequence,infrared spectra obtained by the analyzer 60 from the several sensors 26(which may number in the hundreds) provide information on kineticchanges in chemical constituents and molecular structures of thepolyimide resin as it is being cured. Process control adjustments aremade on a real time basis as a function of the analytical informationobtained from the analyzer spectra so that the resulting laminates areof greater uniformity and quality.

While particular embodiments of the invention have been shown anddescribed, other embodiments will be apparent to those skilled in theart, and therefore it is not intended that the invention be limited tothe disclosed embodiments or to details thereof, and departures may bemade therefrom within the spirit and scope of the invention.

What is claimed is:
 1. An infrared spectroscopy system for monitoringthe processing of a polymeric material comprisinga source of infraredradiation for generating a beam of infrared radiation, infrared spectrumanalyzing means, an infrared radiation transmission fiber that has atransmission portion and a sensor portion adapted to be embedded in thepolymeric material to be monitored, and means for coupling saidtransmission fiber to said source to transmit a beam of infraredradiation through said fiber to said sensor portion and for couplingsaid fiber to said infrared spectrum analyzing means for analyzing theresulting spectra as said polymeric material is processed to providekinetic information on the processing of said polymeric material.
 2. Thesystem of claim 1 wherein said source is of the Michelson interferometertype.
 3. The system of claim 1 wherein said spectrum analyzing means isof the Fourier transform type.
 4. The system of claim 1 wherein thematerial of said fiber is selected from the class consisting ofchalcogenide glass such as arsenic sulfide or arsenic germaniumselenide, heavy metal fluoride glass, such as a mixture of zirconium,barium, lanthanum and aluminum fluorides, and polycrystalline or singlecrystal materials such as thallium bromoiodide or cesium iodide.
 5. Thesystem of claim 1 wherein said sensor .[.fiber.]. .Iadd.portion.Iaddend.has a diameter of at least about one hundred micrometers and arefractive index greater than 1.7.
 6. The system of claim 1 wherein said.[.sensor-transmitter.]. .Iadd.transmission .Iaddend.fiber has a lengthof at least about two meters with a clad transmission portion and anunclad sensor portion, both of which are adapted to be embedded in thepolymer material to be monitored.
 7. The system of claim 6 wherein saidfiber has a diameter of at least about one hundred micrometers, arefractive index greater than 1.7 and an overall transmission loss ofless than 5 dB per meter over an 1800-750 wavenumber bandwidth and saidsensor portion of said fiber has a length that is less than about fivepercent of the overall length of said fiber.
 8. The system of claim 7wherein the material of said fiber is selected from the class consistingof chalcogenide glass such as arsenic sulfide or arsenic germaniumselenide, heavy metal fluoride glass, such as a mixture of zirconium,barium, lanthanum and aluminum fluorides, and polycrystalline or singlecrystal materials such as thallium bromoiodide or cesium iodide.
 9. Thesystem of claim 1 wherein said .[.sensor-transmitter.]..Iadd.transmission .Iaddend.fiber has an overall transmission loss ofless than 5 dB per meter over an 1800-750 wavenumber bandwidth.
 10. Aprocess for infrared spectroscopic monitoring of insitu compositionalchanges in a polymeric material comprising the steps ofproviding anelongated infrared radiation transmitting fiber that has a transmissionportion and a sensor portion, embedding said sensor portion in thepolymeric material to be monitored, subjecting said polymeric materialto a processing sequence, applying a beam of infrared radiation to saidfiber for transmission through said transmitting portion to said sensorportion for modification as a function of properties of the polymericmaterial, and monitoring the modified infrared radiation spectra as saidpolymeric material is being subjected to said processing sequence toobtain kinetic data on changes in the polymeric material during theprocessing sequence.
 11. The process of claim 10 and further includingthe step of adjusting the processing sequence as a function of thekinetic data provided by said modified infrared radiation spectrainformation.
 12. The process of claim 10 wherein a polymer-fiber matrixis being monitored, said polymer-fiber matrix including alternatinglayers of fibers and polymer resins, and the sensor portions of aplurality of said infrared radiation transmitting fibers are embedded inspaced relation in said polymer resin layers.
 13. The process of claim12 wherein said polymeric material being monitored is selected from theclass consisting of epoxies and polyimides.
 14. The process of claim 13wherein the material of said infrared radiation transmitting fibers isselected from the class consisting of chalcogenide glass such as arsenicsulfide or arsenic germanium selenide, heavy metal fluoride glass, sucha mixture of zirconium, barium, lanthanum and aluminum fluorides, andpolycrystalline or single crystal materials such as thallium bromoiodideor cesium iodide.
 15. A elongated infrared radiation transmittingprocess monitoring fiber for use in the process of claim 10, said fiberhaving a transmission portion and a sensor portion, said fiber having alength of at least about one meter and said sensor portion having alength that is less than about five percent of the overall length ofsaid fiber.
 16. The fiber of claim 15 wherein said fiber has an overalltransmission loss of less than 5 dB per meter over an 1800-750wavenumber bandwidth, a diameter of at least about one hundredmicrometers, a refractive index greater than 1.7, said sensor portion isunclad, and said transmission portion is clad.
 17. The fiber of claim 16wherein the material of said fiber is selected from the class consistingof chalcogenide glass such as arsenic sulfide or arsenic germaniumselenide, heavy metal fluoride glass, such a mixture of zirconium,barium, lanthanum and aluminum fluorides, and polycrystalline or singlecrystal materials such as thallium bromoiodide or cesium iodide.
 18. Asystem for insitu infrared spectroscopic monitoring of a polymer-fibermatrix to provide kinetic information during the curing of the polymericconstituent of said polymeric fiber matrix, said polymer-fiber matrixincluding alternating layers of fibers and polymer resins, comprisingaplurality of infrared radiation transmitting fibers, each said.Iadd.transmitting .Iaddend.fiber having a transmission portion and asensor portion, the sensor portions of said .Iadd.transmitting.Iaddend.fibers being adapted to be embedded in spaced relation in saidlayers of polymer resin, a source of infrared radiation for generating abeam of infrared radiation, means for coupling said transmitting fibersto said source to transmit infrared radiation through said fibers tosaid sensor portions, infrared spectrum analyzing means, and means forcoupling said .Iadd.transmitting .Iaddend.fibers to said infraredspectrum analyzing means for analyzing the resulting infrared spectra assaid polymer resin constituents are being cured to provide kineticinformation on the curing of said polymer resins.
 19. The system ofclaim 18 wherein each said infrared radiation transmitting fiber has adiameter of at least about one hundred micrometers and a length of atleast about one meter, each said transmission portion is clad and eachsaid sensor portion is unclad, each said sensor portion has a lengththat is less than about five percent of the overall length of its.Iadd.transmitting .Iaddend.fiber.
 20. The system of claim 19 whereinthe material of said infrared radiation transmitting fibers is selectedfrom the class consisting of chalcogenide glass such as arsenic sulfideor arsenic germanium selenide, heavy metal fluoride glass, such amixture of zirconium, barium, lanthanum and aluminum fluorides, andpolycrystalline or single crystal materials such as thallium bromoiodideor cesium iodide; said polymer-fiber matrix includes fiber of materialsuch as graphite or boron and polymers of materials such as epoxies orpolyimides; each said infrared radiation transmitting fiber has arefractory index greater than 1.7 and an overall transmission loss ofless than 5 dB per meter over an 1800-750 wavenumber bandwidth; andfurther including autoclave type containment structure for receivingsaid polymer-fiber matrix during cure, said containment structureincluding gland structure through said transmission portions of said.Iadd.transmitting .Iaddend.fibers are connected to said infraredradiation source and said infrared spectrum analyzer means. .Iadd.
 21. Asystem for detecting chemical characteristics within a materialcomprising:an optical fiber embedded in a material, the fiber having anunclad sensor section; source means connected to direct opticalradiation into the fiber; encoding means to modulate the opticalradiation; and spectrum analyzer means connected to the fiber to detectat least a part of the spectrum of the modulated optical radiation and,thereby, to detect chemical characteristics of the material in which thesensor section is embedded. .Iaddend. .Iadd.22. A system according toclaim 21 wherein the unclad sensor section comprises a section betweenthe ends of the fiber, and said source means is connected to one end ofthe fiber and said spectrum analyzer means is connected to the other endof the fiber. .Iaddend. .Iadd.23. A system according to claim 21 whereinthe unclad sensor section is at the end of the fiber, and said sourcemeans and said spectrum analyzer means are connected to the same end ofthe fiber. .Iaddend. .Iadd.24. A system according to claim 21 whereinsaid optical fiber extends outside said spectrum analyzer means..Iaddend. .Iadd.25. A system according to claim 23 wherein the tip ofthe unclad sensor section of the fiber is generally planar. .Iaddend..Iadd.26. A system according to claim 21 wherein said spectrum analyzermeans comprises a Fourier transform analyzer. .Iaddend. .Iadd.27. Asystem according to claim 21 wherein said encoding means comprises aninterferometer. .Iaddend. .Iadd.28. A system according to claim 21wherein said unclad sensor section is formed from chalcogenide glass..Iaddend. .Iadd.29. A system according to claim 21 wherein saidchalcogenide glass is arsenic germanium selenide glass. .Iaddend..Iadd.30. A system according to claim 21 wherein said sensor section isformed from metal fluoride glass. .Iaddend. .Iadd.31. A system forproviding in-situ detection of the chemical state within afiber-reinforced composite material, comprising:an optical fiberembedded in the fiber-reinforced composite material, the fiber having anunclad sensor section and the remainder of the fiber being clad; a lightsource connected to direct infrared radiation into the fiber; a detectorconnected to detect infrared radiation emerging from an end of thefiber; and a spectrum analyzer connected to the detector for measuringat least a part of the spectrum of infrared radiation absorbed by thecomposite material adjacent the sensor section. .Iaddend. .Iadd.32. Asystem according to claim 31 wherein the unclad sensor section comprisesa section intermediate the ends of the fiber, and the light source isconnected to one end of the fiber and the spectrum analyzer is connectedto the other end of the fiber. .Iaddend. .Iadd.33. A system according toclaim 31 wherein the unclad sensor section is at the end of the fiber..Iaddend. .Iadd.34. A system according to claim 33 wherein the lightsource and the spectrum analyzer are in optical communication with thesame end of the fiber. .Iaddend. .Iadd.35. A system according to claim31 wherein the spectrum analyzer comprises a Fourier transform analyzer..Iaddend. .Iadd.36. A system according to claim 31 wherein said lightsource includes an interferometer. .Iaddend. .Iadd.37. A process fordetecting chemical characteristics within a material, comprising thesteps of:forming an optical fiber having an unclad sensor section;embedding the sensor section of the optical fiber in a material to beanalyzed; directing light into the fiber; and operating a spectrumanalyzer connected to the fiber to analyze at least a part of thespectrum of said light absorbed by the material adjacent the sensorsection. .Iaddend. .Iadd.38. A process according to claim 37 wherein thematerial is a thermosettable matrix resin. .Iaddend. .Iadd.39. A processaccording to claim 38 wherein the analyzer is operated while curing theresin. .Iaddend. .Iadd.40. A process according to claim 39 wherein theanalyzer measures the degree of cross-linking of the resin. .Iaddend..Iadd.41. A process according to claim 37 wherein a plurality of theoptical fibers, each having a sensor section, are embedded in thematerials at selected locations. .Iaddend. .Iadd.42. A process accordingto claim 37 wherein the material to be analyzed is located external ofthe analyzer. .Iaddend. .Iadd.43. A process according to claim 42wherein the analyzer is a Fourier transform infrared analyzer. .Iaddend..Iadd.44. A process according to claim 43 wherein the analyzer radiationis in the mid-radiation spectrum. .Iaddend. .Iadd.45. A processaccording to claim 44 wherein the wavelengths of analyzed radiationranges from about five to about fifteen microns. .Iaddend. .Iadd.46. Aprocess according to claim 45 wherein the spectrum analysis isaccomplished by measuring multiple internal reflections. .Iaddend..Iadd.47. A process according to claim 37 wherein the unclad sensorsection is connected between the ends of the fiber and light is directedinto one end of the fiber and the spectrum analyzer is connected to theother end of the fiber. .Iaddend. .Iadd.48. A process according to claim37 wherein the unclad sensor section is at the end of the fiber..Iaddend. .Iadd.49. A process according to claim 37 wherein the core ofthe optical fiber is formed from chalcogenide glass. .Iaddend. .Iadd.50.A process according to claim 37 wherein the core of the optical fiber isformed from metal fluoride glass. .Iaddend. .Iadd.51. A process forproviding in-situ detection of chemical conditions within a fiberreinforced composite material during curing, comprising:embedding anoptical fiber in the fiber-reinforced composite material with at leastone end of the fiber extending from the material, the fiber having anunclad sensor section and the remainder of the fiber being clad;connecting a source of infrared radiation to the fiber; and connecting aFourier transform spectrum analyzer to the fiber for detecting thespectrum of infrared radiation absorbed by the composite materialadjacent the unclad sensor section. .Iaddend. .Iadd.52. A processaccording to claim 51 wherein the unclad sensor section is at the end ofthe fiber. .Iaddend. .Iadd.53. A process according to claim 51 whereinthe unclad sensor section is between the ends of the fiber. .Iaddend..Iadd.54. A sensor for use with a spectrum analyzer for detecting thechemical state of materials external to the spectrometer comprising:anoptical fiber having an unclad sensor section for insertion into amaterial whose chemical state is to be measured and an end adapted forconnection to a spectrum analyzer. .Iaddend. .Iadd.55. A sensoraccording to claim 54 wherein the material comprising the sensor sectionis compatible with Fourier transform infrared spectrum analyzer..Iaddend. .Iadd.56. A system for detecting chemical characteristics of amaterial comprisinga source of radiation for generating a beam ofradiation, analyzing means, an optical radiation transmission fiber thathas a transmission portion and a sensor portion adapted to be disposedin said material to be monitored, and means for coupling saidtransmission fiber to said source to transmit a beam of radiationthrough said fiber to said sensor portion and for coupling said fiber tosaid analyzing means for detecting chemical characteristics of saidmaterial. .Iaddend. .Iadd.57. The system of claim 56 wherein thematerial of said transmission portion of said optical radiationtransmission fiber is different from the material of said sensorportion. .Iaddend. .Iadd.58. A system for providing insitu detection ofthe chemical state of a fiber-reinforced composite material comprisinganoptical fiber, said fiber having a transmission portion and a sensorportion, the sensor portion of said fiber being adapted to be disposedin said fiber-reinforced composite material, a source of radiation forgenerating a beam of radiation, means for coupling said optical fiber tosaid source to transmit radiation through said fiber to said sensorportion, spectrum analyzing means, and means for coupling said fiber tosaid spectrum analyzing means for measuring at least a part of theradiation absorbed by the composite material adjacent said sensorportion. .Iaddend.